The Food and Drug Administration has opened an investigation into a range of brands that offer wet dog food. (AP Photo/Mary Altaffer)

The Food and Drug Administration is investigating​ dog food that tested positive for traces of a euthanasia drug.

​The findings may indicate that companies are using euthanized animals to make their dog food, a practice that is illegal. ​Federal law also states that the drug, pentobarbital, cannot appear in any food.

Pentobarbital is used to euthanize animals and has also been used in doctor-assisted death as well as in carrying out the death penalty.

​The dog food, called Gravy Train, showed trace​s pentobarbital across more than two dozen brands of 62 wet dog food samples, according to Ellipse Analytics, the laboratory that carried out the testing. The firm was hired for the investigation by the Washington, D.C.-based news organization WJLA.